The invention relates generally to optoelectronic devices and more particularly to maintaining source-to-lens alignment along three perpendicular axes.
Transmitting data using optical signals is increasingly taking the place of the traditional approach of exchanging data via electrical signals. An optoelectronic module provides the interface between an optical transfer medium and an electrical medium. For example, the optical transfer medium may be a fiber cable that terminates with a connector that exposes ends of an array of optical fibers. Laser diodes, such as Fabry-Perot lasers or Vertical Cavity Surface Emitting Lasers (VCSELs), are commonly used to generate optical signals in response to electrical excitation signals. Laser diodes are preferred in many applications, since they provide high performance signaling in a miniaturized environment.
FIG. 1 illustrates key components of an optoelectronic system. In the illustrated embodiment, the system is a twelve-channel parallel fiber arrangement. Light sources 10, such as VCSELs, are fabricated on a substrate 12. The substrate may be a semiconductor die, such as a gallium arsenide chip. A lens array 14 resides between the light sources and an array of parallel optical fibers 16. The lens array is shown as including a number of optical elements 18, which are used to manipulate light rays passing from the sources 10 to the fibers 16. For example, the optical elements may be diffractive elements.
While not shown in FIG. 1, an optoelectronic module includes hardware components that secure the light sources 10, the lens array 14, and the optical fibers 16. As is well known in the art, the optical components should be aligned along x and y axes to ensure integrity of signal exchanges. Often, guide pins are used to provide the alignment. For example, guide pins extending along the z axis may have central regions that pass through the lens array 14, so that end portions can extend into both the substrate 12 and the removable connector that supports the optical fibers 16. U.S. Pat. No. 5,917,976 to Yamaguchi describes an optical transmission path coupling apparatus that includes guide pins and guide pin holes to provide alignment of fibers to microlenses and light receivers/emitters, with the alignment being along the x and y axes. U.S. Pat. No. 5,867,621 to Luther et al. describes the use of guide pins to properly position two optical fiber connectors, so that the fibers of the connectors are aligned along the x and y axes.
Alignment along the z axis is also important to achieving desired performance in a high speed application. In one example, the desired distance between the light sources 10 and the optical elements 18 may be 2.0 millimeters, with a tolerance of xc2x135 microns in order to pass a sufficient percentage of emitted light to maintain performance. Z-axis alignment is set in some products by lowering the lens array 14 over the array of light sources 10 while sensing the light that is transmitted through the optical elements 18. The lens array is fixed in position relative to the light sources when maximum light is transmitted through the optical elements.
Another z-axis alignment that is critical to optimal performance is the alignment of the ends of the fibers 16 from the optical elements 18. As one example, the target distance may be 0.475 millimeters, with a tolerance of xc2x125 microns. This may be achieved by using an alignment tool to join a connector receptacle to another component of the optoelectronic module to which the lens array 14 is attached.
While the use of known alignment tools and procedures may provide the target results, the process is often time consuming, so that production throughput is lowered. What is needed is a system and method that provide repeatable precision alignments for an optoelectronic module, with alignments along three axes being achieved without the need of alignment fixtures.
An optical coupling system utilizes a one-piece, separation-setting member for defining precise spatial relationships from a lens array to both an array of light sources and an array of optical fibers. The optical fibers are arranged along an end face of a fiber connector that abuts an exterior connector-contacting surface of the separation-setting member. The connector-contacting surface is configured to locate and align the optical fibers along a xe2x80x9ctargetxe2x80x9d plane.
The separation-setting member includes an interior region in which the lens array resides. The lens array abuts a shoulder having a precisely controlled distance from the target plane having the ends of the optical fibers when the fiber connector is seated against the connector-contacting surface. This precisely controlled distance is based upon maximizing the light transfer through the optical lenses of the lens array to the fibers. The lens array is seated in a manner in which it is parallel to the target plane and is exposed to the target plane through an opening within the separation-setting member.
The separation-setting member also includes a back surface that has a precisely controlled distance from the shoulder against which the lens array is seated. During assembly, the back surface is positioned against a substrate that supports the array of light sources. For example, the substrate may be a flex circuit having conductive traces to a semiconductor chip on which light sources, such as VCSELs, are integrated. Because the back surface is at the precisely controlled distance from the shoulder and because the back surface is parallel to the shoulder and the target plane, the light sources will have a desired orientation and distance relative to the lens array. In the preferred embodiment, the portion of the back surface that abuts the light source-supporting substrate is comprised of a number of feet that are strategically positioned to ensure that the parallelism is maintained while providing some access to the interior for a bonding step.
An advantage of the invention is that fabricating the separation-setting member to tight tolerances enables the spatial relationships to be achieved without the use of special z-axis alignment steps or tools. Axial alignments along x and y axes are achieved using conventional techniques. For example, the lens array is precisely positioned along the shoulder using a visual alignment system prior to gluing the lens array to the shoulder. Subsequently, x-direction alignment and y-direction alignment between the lens array and the array of light sources may be achieved using active alignment in which power through the lenses from the light sources is monitored while the relative positioning of the two arrays is stepped in increments of one micron. The separation-setting member is fixed in the position at which power is at a maximum. Guide pins are used to provide x and y axes alignment of the connector. It should be noted that the use of guide pins requires exacting positional tolerances of the guide pin holes. The guide pins should extend through the separation-setting member into holes of both the fiber connector and the light source-supporting substrate.
The preferred embodiment of the optical coupling system includes a connector receptacle that releasably attaches the fiber connector such that the fiber ends are aligned along the target plane. That is, the receptacle should position the connector to abut the separation-setting member. In this preferred embodiment, the receptacle has both a locked position and a release position relative to the separation-setting member. In the locked position, the receptacle physically engages the separation-setting member, so that the one-piece components are in a desired orientation. However, by rotating the receptacle, the receptacle is moved to its release position in which it can be removed from the separation-setting member.
In accordance with the method, the lens array is seated within the interior region of the separation-setting member, so as to contact the shoulder. The back surface of the separation-setting member is then placed in contact with the substrate on which the light sources reside. In one example of the method, the lens array is 2.0 millimeters from VCSELs fixed to a flexible circuit. The distance from the VCSELs to the array is maintained within a tolerance of 35 microns by forming the separation-setting member with a shoulder-to-back surface tolerance of xc2x110 microns. Moreover, the parallelism of the shoulder and the rear surface is maintained by the fabrication processing during the formation of the separation-setting member.
The connector receptacle is rotated into its locking position onto the front surface of the separation-setting member. In this position, insertion of a fiber connector into the receptacle places the connector end in abutment with the connector-contacting surface of the separation-setting member. Thus, the fiber ends are aligned along the target plane that is at a controlled distance from the lens array. In one embodiment, the connector is a Mechanical Transfer Plug (MTP) connector that is spring biased into contact with the separation-setting member. The use of MTP connectors and other spring-biased connectors is well known in the art. The distance between the fiber ends and the lens array may be held to 0.475 millimeters, xc2x125 microns, using the invention.
In one embodiment, the optical coupling system also includes a housing having a first portion adapted to receive the substrate after it has been connected to a heatsink or other component. A second portion of the housing is adapted to securely hold the connector receptacle. For example, the connector receptacle may be simultaneously locked to the separation-setting member and the housing by rotating the receptacle from its release position to its locking position. In a final step, the assembled system is coated with an adhesive to provide extra strength. An advantage of the housing is that it eliminates the need for fixturing to hold the parts together while they are being glued and cured.